Field of the Invention
The present invention relates to a wireless receiver that receives a radio wave from, for example, a wireless microphone and demodulates an audio signal, in particular, to the wireless receiver which can appropriately display a radio wave reception state on an indicator.
Description of the Related Art
A wireless microphone and a wireless receiver are normally used in one-to-one relation. That is, in multi-wave operation in which a plurality of wireless microphones is simultaneously used on a stage or the like, the same number of wireless receivers tuned with transmission frequencies of the respective wireless microphones are used.
In this case, according to Japanese Radio Law, for example, as a frequency band that can be used for a type B wireless microphone, 30 channels are allocated for a specified low power radio device, in which a range of 806.125 to 809.750 MHz is equally divided in units of 0.125 MHz.
Therefore, in a case where the plurality of wireless microphones is simultaneously used as described above, it is necessary to set an oscillation frequency of each of the wireless microphones so that the wireless receiver is not influenced by co-channel disturbance, adjacent-channel disturbance, or intermodulation disturbance, and that an operation is necessary to match a reception frequency of the wireless receiver to be paired with the oscillation frequency of each of the wireless microphones.
By the way, a reception state of the radio wave received in each wireless receiver can be known by a reception strength (also referred to as a reception level) displayed on an indicator. What is displayed on the indicator is preferably in proportion to the received radio wave strength ideally.
However, in a case where the received radio wave strength is too weak or too strong, the magnitude of the reception strength may not be displayed on the indicator correctly.
This is because detection sensitivity of a conventional wireless receiver has a dynamic range of about 70 dB at the maximum, and cannot cover a dynamic range of 100 dB which is a range of the generally used wireless receiver from minimum sensitivity 20 dBμVemf to maximum sensitivity 120 dBμVemf. That is, in a weak electric field or a strong electric field, there has been a problem that accurate detection cannot be made for the radio wave strength below or above a predetermined setting range, and the indication is fixed.
A patent document JP 58-87924 A discloses a signal generation system for signal strength display that generates an out-of-range detection signal when an output level (output voltage) corresponding to an input signal for displaying signal strength is outside a predetermined range, and compresses the output level with a predetermined characteristic so as to fall within the predetermined range.
The document discloses that the signal generation system for signal strength indicator outputs a highly accurate DC level proportional to a logarithm of an input level of a wide dynamic range, and enables to display an accurate signal strength. According to a configuration of an invention disclosed in JP 58-87924 A, an output level of magnitude proportional to received signal strength can be obtained as long as the output level of the input signal for displaying the signal strength is within the predetermined range.
However, since the output level of the input signal is compressed when the output level exceeds the predetermined range, the output level accurately proportional to the magnitude of the reception level has not been obtained.